Toll-like receptors (TLRs) are a family of highly conserved pattern recognition receptor (PRR) polypeptides that recognize pathogen-associated molecular patterns (PAMPs) and play a critical role in innate immunity in mammals. Currently at least ten family members, designated TLR1-TLR10, have been identified. The cytoplasmic domains of the various TLRs are characterized by a Toll-interleukin 1 receptor (TIR) domain. Medzhitov R et al. (1998) Mol Cell 2:253-8. Recognition of microbial invasion by TLRs triggers activation of a signaling cascade that is evolutionarily conserved in Drosophila and mammals. The TIR domain-containing adapter protein MyD88 has been reported to associate with TLRs and to recruit interleukin 1 receptor-associated kinase (IRAK) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) to the TLRs. The MyD88-dependent signaling pathway is believed to lead to activation of NF-κB transcription factors and c-Jun NH2 terminal kinase (Jnk) mitogen-activated protein kinases (MAPKs), critical steps in immune activation and production of inflammatory cytokines. For reviews, see Aderem A et al. (2000) Nature 406:782-87, and Akira S et al. (2004) Nat Rev Immunol 4:499-511.
A number of specific TLR ligands have been identified. Ligands for TLR2 include peptidoglycan and lipopeptides. Yoshimura A et al. (1999) J Immunol 163:1-5; Yoshimura A et al. (1999) J Immunol 163:1-5; Aliprantis A O et al. (1999) Science 285:736-9. Lipopolysaccharide (LPS) is a ligand for TLR4. Poltorak A et al. (1998) Science 282:2085-8; Hoshino K et al. (1999) J Immunol 162:3749-52. Bacterial flagellin is a ligand for TLR5. Hayashi F et al. (2001) Nature 410:1099-1103. Peptidoglycan has been reported to be a ligand not only for TLR2 but also for TLR6. Ozinsky A et al. (2000) Proc Natl Acad Sci USA 97:13766-71; Takeuchi O et al. (2001) Int Immunol 13:933-40. Recently certain low molecular weight synthetic compounds, the imidazoquinolines imiquimod (R-837) and resiquimod (R-848), were reported to be ligands of TLR7 and TLR8. Hemmi H et al. (2002) Nat Immunol 3:196-200; lurk M et al. (2002) Nat Immunol 3:499.
Beginning with the discovery that unmethylated bacterial DNA and synthetic analogs thereof (CpG DNA) are ligands for TLR9 (Hemmi H et al. (2000) Nature 408:740-5; Bauer S et al. (2001) Proc Natl Acad Sci USA 98, 9237-42), it has been reported that ligands for certain TLRs include certain nucleic acid molecules. Recently it has been reported that certain types of RNA are immunostimulatory in a sequence-independent or sequence-dependent manner. Further, it has been reported that these various immunostimulatory RNAs stimulate TLR3, TLR7, and TLR8.